Three phase Wye Transformer KVA calculation/ Homework typo?

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JasonCo

Senior Member
Location
Houston, Texas
Three phase loads of 16, 42, and 75 amps are fed from a 208-volt panel. The smallest Group D transformer required to feed the 120/208-volt 3-ph, 4-wire panel has a rating of ___ KVA.


Group D transformer KVA ratings:
3KVA, 6KVA, 9KVA, 15KVA, 25KVA, 30KVA, 37.5KVA, 45KVA, 50KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1000KVA


So this is what I thought I would do...
75 + 42 + 16 = 133amps
3phase power = 1.732 x 133 x 208 = 47914 = 47.9KVA


My answer choices are:
A. 150
B. 225
C. 45
D. 300
E. 112.5
F. 75


There is no answer... Because looking at the Group D transformers, the next one up that I need is rating for 50 KVA, which is not one of the answer choices.. Have I gone crazy or is this just a homework typo?
 

Smart $

Esteemed Member
Location
Ohio
Perhaps a typo. Possibly looking for the most correct of the choices. Can't decide which from here.

:angel:
 

kwired

Electron manager
Location
NE Nebraska
Three phase loads of 16, 42, and 75 amps are fed from a 208-volt panel. The smallest Group D transformer required to feed the 120/208-volt 3-ph, 4-wire panel has a rating of ___ KVA.


Group D transformer KVA ratings:
3KVA, 6KVA, 9KVA, 15KVA, 25KVA, 30KVA, 37.5KVA, 45KVA, 50KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1000KVA


So this is what I thought I would do...
75 + 42 + 16 = 133amps
3phase power = 1.732 x 133 x 208 = 47914 = 47.9KVA


My answer choices are:
A. 150
B. 225
C. 45
D. 300
E. 112.5
F. 75


There is no answer... Because looking at the Group D transformers, the next one up that I need is rating for 50 KVA, which is not one of the answer choices.. Have I gone crazy or is this just a homework typo?
I don't know exactly what group D means, but all the sizes you mentioned are standard sizes, however some of them are only common sizes for single phase, some of them for three phase.

On the lower capacity end of what you mentioned the 25, 37.5 and 50 are usually only seen as single phase transformers, which would leave you with 75 being next higher three phase rating over your load.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I concur with your math. In a real world, I would want to know the nature of the three loads. Are they motors, and should I therefore include a 125% factor? Are they pumps that run intermittently, so that I don't have to include all the load at the same time? Are there any other loads, or do I need to account for future load additions? From a homework or test perspective, however, I think they chose numbers that would go above the 45 KVA transformer rating, and make you struggle to infer whether using a 75 KVA would be overkill. My answer would be 75 KVA, because that is the next higher standard rating, and because the question asked for the smallest required size.
 

JasonCo

Senior Member
Location
Houston, Texas
I concur with your math. In a real world, I would want to know the nature of the three loads. Are they motors, and should I therefore include a 125% factor? Are they pumps that run intermittently, so that I don't have to include all the load at the same time? Are there any other loads, or do I need to account for future load additions? From a homework or test perspective, however, I think they chose numbers that would go above the 45 KVA transformer rating, and make you struggle to infer whether using a 75 KVA would be overkill. My answer would be 75 KVA, because that is the next higher standard rating, and because the question asked for the smallest required size.

Great responses from everyone! Yes I decided that 75KVA would be the only answer that makes sense. 45KVA is too low... Just wish they had put 50KVA in the answer choices :) Because that is what the question leads me to believe the answer should be. No point in using a 75KVA when a 50KVA is all I need. But yes I'm 100% agree with you, that all these questions it would be nicer if they specified if it is a heater motor or something, or a continuous and etc... But they are all pretty straight forward, which in some cases makes it hard for me to answer the questions. Anyways, teacher gave us 100 question homework and each question involves math. Really enjoy this stuff though, almost done with the homework! Thanks for all your help
 

kwired

Electron manager
Location
NE Nebraska
Great responses from everyone! Yes I decided that 75KVA would be the only answer that makes sense. 45KVA is too low... Just wish they had put 50KVA in the answer choices :) Because that is what the question leads me to believe the answer should be. No point in using a 75KVA when a 50KVA is all I need. But yes I'm 100% agree with you, that all these questions it would be nicer if they specified if it is a heater motor or something, or a continuous and etc... But they are all pretty straight forward, which in some cases makes it hard for me to answer the questions. Anyways, teacher gave us 100 question homework and each question involves math. Really enjoy this stuff though, almost done with the homework! Thanks for all your help

But as I mentioned earlier you are not all that likely to find a 50 kVA three phase transformer unless it is custom made for you. 50 is a common single phase size though.
 

topgone

Senior Member
Three phase loads of 16, 42, and 75 amps are fed from a 208-volt panel. The smallest Group D transformer required to feed the 120/208-volt 3-ph, 4-wire panel has a rating of ___ KVA.


Group D transformer KVA ratings:
3KVA, 6KVA, 9KVA, 15KVA, 25KVA, 30KVA, 37.5KVA, 45KVA, 50KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1000KVA


So this is what I thought I would do...
75 + 42 + 16 = 133amps
3phase power = 1.732 x 133 x 208 = 47914 = 47.9KVA


My answer choices are:
A. 150
B. 225
C. 45
D. 300
E. 112.5
F. 75


There is no answer... Because looking at the Group D transformers, the next one up that I need is rating for 50 KVA, which is not one of the answer choices.. Have I gone crazy or is this just a homework typo?


Take it like this: You are given per phase amps and you need to size a transformer to serve those loads and not get overloaded! You have a max load of 75A, and you need to find a transformer that when loaded with 75 amperes on one phase runs fine. So:
kVA required = 75 x 1.732 x 208 = 27 kVA!
The nearest standard transformer size should be 30 kVA! But on your list, you are given 45 kVA!
 

kwired

Electron manager
Location
NE Nebraska
Take it like this: You are given per phase amps and you need to size a transformer to serve those loads and not get overloaded! You have a max load of 75A, and you need to find a transformer that when loaded with 75 amperes on one phase runs fine. So:
kVA required = 75 x 1.732 x 208 = 27 kVA!
The nearest standard transformer size should be 30 kVA! But on your list, you are given 45 kVA!
What if the mentioned 16 and/or 42 amp loads are to run at same time as the 75 amp load?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
To add the currents arithmetically, they would all need to be in phase but that piece of information seems to me missing.
No, it is there. All three of the loads are 3-phase loads, and therefore they all in phase with each other. The inherent assumption is that each load is balanced within itself. But given that, we can add amps to amps and get a correct answer in amps. Not a good general practice, mind you. But this time it works.

 

Smart $

Esteemed Member
Location
Ohio
No, it is there. All three of the loads are 3-phase loads, and therefore they all in phase with each other. The inherent assumption is that each load is balanced within itself. But given that, we can add amps to amps and get a correct answer in amps. Not a good general practice, mind you. But this time it works.

What if each motor operates at a different power factor?

What you're saying would only be true if all three motors operate at the same power factor.
 

Sahib

Senior Member
Location
India
No power factor is given in the problem. As such, charlie's approach is correct and also applicable in other practical estimates of transformer capacity for loads of known KVA but unknown power factor.
 

Smart $

Esteemed Member
Location
Ohio
No power factor is given in the problem. As such, charlie's approach is correct and also applicable in other practical estimates of transformer capacity for loads of known KVA but unknown power factor.
I agree with this but not how he stated all three loads will be in phase. This works where no power factor is given because no matter the power factor, the combined current magnitude can never exceed the arithmetic sum of currents.
 

Smart $

Esteemed Member
Location
Ohio

Sahib

Senior Member
Location
India
The two threads are interconnected in that currents, in a phase are to be arithmetically summed up to find the solution.
 
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